Fasting can be a useful tool for weight loss and better health. Many healthy adults can fast without serious problems. 

But fasting isn’t right for everyone. Your health, medications, and the length of your fast all play a role in how safe it is. 

Whether you’re trying Intermittent Fasting or a long Extended Fast, it’s important to know the fasting risks. 

Who should avoid fasting?

Fasting can be risky for some people. These groups should avoid fasting for weight loss unless their healthcare provider gives them specific advice. 

Pregnant or breastfeeding women

Your body needs extra nutrients during pregnancy. These nutrients help your baby grow.  Breastfeeding also takes extra energy. Fasting can make it harder to get what you need. 

There isn’t enough research to show that fasting is safe during these times, so it’s best avoided. 

Women actively trying to conceive

Women who are trying to become pregnant should speak with their healthcare provider before starting a fasting plan, particularly an Extended Fast or a restrictive eating schedule. 

Getting enough nutrients is important for reproductive health, and strict fasting may not be the best choice. 

Children and teenagers

Children and teenagers need enough energy and nutrients to support growth, development, and hormonal changes. Restricting their eating through fasting can make it harder to meet these needs. 

Fasting is generally not recommended for people under 18. 

People with a history of eating disorders

Fasting introduces rules around when you can and cannot eat. 

For people with a history of anorexia, bulimia, binge eating, or other eating disorders, these rules may trigger harmful behaviours around food. 

Even if fasting has helped with weight management in the past, it’s important to consider your relationship with food. If fasting leads to obsessive thoughts about eating, cycles of restriction and overeating, or intense guilt around food, it may not be the right approach. 

Who should get medical advice before fasting?

Some people may be able to fast, but certain health conditions and medications can make fasting risky. 

Talk to your healthcare provider about fasting risks before starting if you have an ongoing health problem or are planning an Extended Fast. 

People with diabetes or blood sugar problems

Fasting changes how your body regulates blood glucose. For people taking insulin or certain diabetes medications, going without food can cause hypoglycemia, or dangerously low blood sugar. 

That doesn’t mean that people with diabetes can never fast. 

But they should discuss fasting with their healthcare team before starting. They may need medication adjustments and a plan for monitoring blood glucose. 

Never change your medication dose on your own. 

People with kidney disease

Your kidneys help keep the right balance of fluids and minerals in your body. Kidney disease can make it harder for your body to keep this balance, increasing the risk of dehydration and mineral imbalances.

Some people with kidney disease also have specific fluid or mineral restrictions, and that has to be taken into consideration. 

People with high blood pressure and cardiovascular conditions

Fasting can affect blood pressure, particularly when fluid intake is inadequate. People taking blood pressure medications or water pills may be more likely to experience low blood pressure, dizziness, and dehydration. 

Heart conditions can also make fluid and electrolyte changes more concerning. 

Electrolytes such as sodium and potassium help regulate nerve signals, muscle contractions, and heart rhythm. Both too little and too much can cause problems, especially in people with certain medical conditions or medications. 

People who take prescription medications

Some medications need to be taken with food, while others affect blood sugar, blood pressure, kidney function, and fluid balance. Going without food can change how some medications work in your body. 

Before starting, ask your healthcare provider or pharmacist whether your medication schedule is compatible with fasting. This is especially important for Extended Fasting, during which the effects of going without food and changes in fluid intake may be greater. 

What are the potential risks of fasting?

Even when fasting is appropriate, side effects can occur. 

Knowing which symptoms to watch for can help you stay safe while fasting.

• Dehydration. Food provides some of the fluid we take in each day. When you stop eating, you lose that source of fluid, and changes in insulin levels can also increase water and sodium loss. 

• Electrolyte imbalances. Sodium, potassium, and magnesium help your nerves, muscles, and heart work properly. Changes in fluid levels can upset this balance and cause problems such as  muscle weakness, confusion, or abnormal heart rhythms. 

• Hypoglycemia. Symptoms such as shakiness, sweating, weakness, and confusion should never be ignored. 

• Low blood pressure, fatigue, and dizziness. Changes in hydration and blood pressure can leave you feeling weak or lightheaded. 

Research on Intermittent Fasting has found that it is generally well tolerated by many adults who are overweight or obese. A 2024 review of 15 studies involving 1,365 adults found no major increase in fatigue, headaches, or dropout rates compared with control groups. 

Warning signs

It’s important to listen to your body rather than pushing through symptoms to reach a target fasting time. 

Hunger and cravings are normal, but you should stop your fast and seek medical advice if you experience:

• Persistent dizziness or lightheadedness

• Shakiness, sweating, unusual weakness, or a racing heartbeat 

• Severe or persistent headache, nausea, or vomiting 

• Signs of dehydration

• Unusual fatigue 

Seek urgent medical attention for fainting, confusion, chest pain, severe shortness of breath, seizures, or a sustained irregular heartbeat. These can be signs of a serious problem. 

Safety comes first

Interestingly, some of the conditions mentioned above, such as type 2 diabetes, may improve with fasting. But fasting has to be practiced safely and with proper guidance. If you have a medical condition, or are considering an Extended Fast, speak to your healthcare provider first. 

And if you develop warning signs, stop fasting. Protecting your health will always matter more than completing a fast. 

References:

https://idf.org/managing-diabetes/fasting/

Zhong F, Zhu T, Jin X, Chen X, Wu R, Shao L, Wang S. Adverse events profile associated with intermittent fasting in adults with overweight or obesity: a systematic review and meta-analysis of randomized controlled trials. Nutr J. 2024 Jul 10;23(1):72. doi: 10.1186/s12937-024-00975-9. PMID: 38987755; PMCID: PMC11234547.